Currently I'm working my way through Mark McMenamin's book The Garden of Ediacara, an analysis of the fossil evidence from the Vendian Period, the last bit of the Precambrian (650-543 million years ago).
The subject of McMenamin's book is undeniably fascinating -- more about that in a moment -- but it's uneven reading. Part of it is a travelogue of his work in Namibia, Mexico, and Australia, places where there are significant outcrops of late Precambrian sedimentary rocks, but it's obvious from page one that most of what he does is write papers for scholarly journals. As a result, it's halfway between an introduction to the topic for laypeople and an extended academic paper, and I've been glad as I worked my way through it that I have at least a passing background in paleontology.
Something that struck me right away, however, was that I've been laboring under a serious misunderstanding of the Ediacaran biota; that it overlapped significantly with the Cambrian explosion fauna, the bizarre creatures like Anomalocaris and Opabinia and the aptly-named Hallucigenia. In reality, there was almost no overlap, and the Ediacaran organisms such as Cloudina and Dickinsonia were almost certainly driven to extinction and replaced by the large predatory forms of the early Cambrian.
While the early Cambrians (best known from the Burgess Shale formation of British Columbia) are clearly animals, the bizarre Ediacarans are of completely uncertain affinities. When McMenamin wrote his book (1998) there was considerable contention about what they were, with various paleontologists arguing vehemently that they were early animals, fungi, algae, or even giant protists (or protist colonies). Despite the passage of twenty-five years, the issue is still far from settled. Some make persuasive arguments that the Vendian biota doesn't belong to any of the five modern kingdoms of life (animals, plants, fungi, bacteria, and archaea), but are representatives of a completely different lineage, or more than one, that left no descendants at all.
So I'm grateful to McMenamin and his book for clearing up something I'd misunderstood for years.
I was in the middle of reading The Garden of Ediacara when, coincidentally, a friend and frequent contributor of topics for Skeptophilia sent me a link to an article in Smithsonian magazine about the evolutionary origin of animals. Another point of contention amongst biologists is determining, out of the entire kingdom Animalia, which group branched off first. (This is sometimes phrased as which is the "oldest" or "most primitive" -- both terminology I don't like, because every living animal on Earth has an exactly equal length of evolutionary history. It's just that during that time, some branches have changed a great deal faster than others, and some groups share more recent common ancestry than others do.)
In any case, the argument is about which group of modern animals is the outgroup -- the one that split off first, and therefore is the most distantly related to all other animals. When I took zoology (many, many years ago) the conventional wisdom was that it was sponges (Phylum Porifera). And there's certainly a good case to be made there; sponges are weird animals, with no differentiated organs, skeletons made of either protein fibers, bits of calcium carbonate, or slivers of glass, and no nerves, muscles, or digestive tracts. But genetic analysis has shown unequivocally that there's an even more distantly-related group -- the comb jellies (Phylum Ctenophora).
They look superficially like jellyfish, and that similarity led scientists to put them on the same branch as Phylum Cnidaria (which not only contains jellyfish, but sea anemones and corals). The genetic studies, though, show that there's only a distant relationship between comb jellies and jellyfish. The comb jellies, in fact, show more of a genetic similarity to certain species of protists than they do to other animals.
"That was the smoking gun," said Daniel Rokhsar, of the University of California - Berkeley, who co-authored the paper.
So this goes to show that there's a lot we still have to learn about the earliest life on our planet. And I'm sure that as definitive as this study seems to be, it won't be the last word. As more evidence surfaces, expect the arrangement to change. This, after all, is how science works; it has a mechanism for self-correcting. And far from the reaction I've seen people have -- that the shifting understanding means "it could all be proven wrong tomorrow" -- that capacity for change is science's main strength.
After all, isn't it a good thing to have your model shift to accommodate new information? Seems like standing firm on what you believe despite strong evidence to the contrary is the cause of a lot of the problems in the world.